1. Field of the Invention
This invention relates generally to radar systems, and more particularly to the detection of an out of beam condition in a monopulse radar receiver.
2. Description of Related Art
In a monopulse radar receiver, sum and difference signals are formed from plural antenna beams. The difference signal is representative of the tracking error, i.e., the angle that the target being tracked is off the boresight line of the antenna and is therefore used for automatic tracking. In some radar systems, usually those employing a single target track mode, a search scan operation precedes the automatic tracking mode. After target acquisition, the search scan mode is stopped and the antenna is trained on the target by the automatic tracking equipment. Other radar systems employ a "track while scan" mode of operation, in which the position of one or more targets is continually tracked on the basis of the data received while the antenna is also performing a search scan. It is necessary that the equipment be able to detect when a target being tracked is out of the beam of the antenna, so that the track can be deleted, in a track while scan system, or the target reacquired by switching to the scanning mode, in a search and track radar.
Current techniques for detecting an out of beam condition are based upon the signal to noise ratio of the radar return signal and do not permit the direction of the target from the boresight to be ascertained. Signal to noise ratio is an ambiguous indicator of an out of beam condition. The signal to noise ratio may drop due to other factors such as ground clutter, noise, jamming or changes in atmospheric condition. To minimize the likelihood of an ambiguity, the signal to noise ratio must be monitored for a long time before a reliable out of beam determination can be made.